The invention concerns an illumination system according to the preamble of claim 1 as well as a projection exposure device with such an illumination system and a process for the production of microelectronic components with a projection exposure device.
In order to still further reduce the structural widths for electronic components, particularly in the submicron range, it is necessary to reduce the wavelength of the light utilized for microlithography. Lithography with soft x-rays, so-called EUV lithography, is conceivable at wavelengths below 193 nm for example.
An illumination system suitable for EUV lithography will homogeneously, i.e., uniformly illuminate, with as few reflections as possible, a predetermined field for EUV lithography, particularly the annular field of an objective. Furthermore, the pupil of the objective should be illuminated up to a specific degree of filling, independent of the field, and the exit pupil of the illumination system should lie in the entrance pupil of the objective.
An illumination system for a lithography device, which uses EUV radiation, has been made known from U.S. Pat. No. 5,339,246. For uniform illumination in the reticle plane and filling of the pupil, U.S. Pat. No. 5,339,246 proposes a condenser, which is constructed as a collector lens, and comprises at least four pairs of mirror facets, which are arranged symmetrically. A plasma light source is used as the light source.
An illumination system with a plasma light source comprising a condenser mirror is shown in U.S. Pat. No. 5,737,137, in which an illumination of a mask or a reticle to be illuminated is achieved by means of spherical mirrors.
U.S. Pat. No. 5,361,292 shows an illumination system, in which a plasma light source is provided and the point plasma light source is imaged by means of a condenser, which has at least three aspherical mirrors arranged off-center, in a ring-shaped illuminated surface. The ring-shaped illuminated surface is then imaged in the entrance pupil by means of a special sequence of grazing-incidence mirrors.
An illumination system has been made known from U.S. Pat. No. 5,581,605, in which a photon beam is spilt into a multiple number of secondary light sources by means of a plate with raster elements. A homogeneous or uniform illumination is achieved in this way in the reticle plane. The imaging of the reticle on the wafer to be exposed is produced by means of a conventional reducing optics. A gridded mirror with equally curved elements is provided precisely in the illuminating beam path.
U.S. Pat. No. 5,677,939 shows an illumination system for EUV illumination devices, in which an annular field is homogeneously illuminated. In the EUV illumination system according to U.S. Pat. No. 5,677,939, the beams emitted from the EUV source are formed into a parallel beam of light, for example, by means of a mirror. In order to form a multiple number of secondary light sources, the parallel beam of light is guided onto a mirror with a plurality of cylinder raster elements. U.S. Pat. No. 5,677,939 also describes the use of synchrotron radiation sources, but of course, the light of the source is guided directly onto the mirror with cylinder raster elements, due to the parallel nature of the emitted synchrotron radiation, without optical elements situated therebetween. All embodiments shown in U.S. Pat. No. 5,677,939 operate in a parallel beam path. In addition, the facetted mirrors known from U.S. Pat. No. 5,677,939 contain facets with an optical effect and are arranged on a planar substrate.
From U.S. Pat. No. 5,512,759 for an arc shaped-field projection system with a synchrotron radiation source an illumination system has been made known, which comprises a condenser system with a multiple number of convergent mirrors. The mirrors collect the radiation emitted by the synchrotron radiation source, to form an annular light beam, which corresponds to the annular field to be illuminated. Therefore, the annular field is illuminated very uniformly. The synchrotron radiation source has a beam divergence  greater than 100 mrads in the beam plane.
U.S. Pat. No. 5,439,781 shows an illumination system with a synchrotron radiation source, in which the waveguide value, i.e., the Lagrange optical invariant, is adjusted by means of a scatter disk in the entrance pupil of the objective, whereby the scatter disk may have a plurality of pyramidal structures. The synchrotron radiation source in the case of U.S. Pat. No. 5,439,781 also has a beam divergence  greater than 100 mrads. The collector mirror for collecting the synchrotron radiation and bundling the same may itself be constructed with facets.
The disclosure content of all of the previously named documents:
U.S. Pat. No. 5,339,246
U.S. Pat. No. 5,737,137
U.S. Pat. No. 5,361,292
U.S. Pat. No. 5,581,605
U.S. Pat. No. 5,677,939
U.S. Pat. No. 5,512,759
U.S. Pat. No. 5,439,781
is incorporated in the present application by reference.
An object of the invention is to provide an illumination system that is constructed as simply as possible fulfilling the requirements for an exposure system for wavelengths xe2x89xa6193 nm, particularly in the EUV region and a process for the design of such a system. In addition to a uniform illumination of the reticle, also the telecentric requirements of a system for wavelengths xe2x89xa6193 nm particularly should be fulfilled.
Telecentricity is to be understood in the present application in that the entire system is telecentric at the wafer. This requires an adaptation of the exit pupil of the illumination system to the entrance pupil of the objective, which is finite for a reflective reticle.
In the present application, the telecentricity requirement is fulfilled, if the divergence of the principal beams of the illumination system and objective in the reticle plane does not exceed a predetermined value, for example, xc2x14.0 mrads, preferably xc2x11.0 mrad, and the principal beams impinge on the wafer telecentrically.
According to the invention, this object is achieved in that for the above-described illumination system, the light source is a light source for producing radiation with a wavelength xe2x89xa6193 nm, which irradiates with a wavelength spectrum in a predetermined plane, wherein the radiation in the wavelength range that can be used for applications, particularly lithography, has a beam divergence perpendicular to the predetermined plane, which is less than 5 mrads.
Synchrotron radiation sources are used in the EUV region as preferred light sources, with a beam divergence smaller than 5 mrads in the plane perpendicular to the predetermined plane. Synchrotron radiation is emitted, if relativistic electrons are deflected in a magnetic field. The synchrotron radiation is emitted tangentially to the path of the electrons.
At the present time, one can distinguish three types of sources in the case of synchrotron radiation sources:
bending magnets
wigglers
undulators.
In bending-magnet sources, electrons are deflected by a bending magnet and photon radiation is emitted.
Wiggler sources contain a so-called wiggler for deflection of the electron or an electron beam, the wiggler comprising many pairs of magnets with alternating polarity arranged in rows. When an electron passes through a wiggler, the electron will be subjected to a periodic, vertical magnetic field; the electron oscillates accordingly in the horizontal plane. Furthermore, wigglers are characterized in that no coherence effects occur. The synchrotron radiation produced by means of a wiggler is similar to that of a bending magnet and radiates in a horizontal solid angle. In contrast to the bending magnet, it has a flux that is amplified by the number of poles of the wiggler.
There is no clear dividing line between wiggler sources and undulator sources.
In the case of undulator sources, the electrons in the undulator are subjected to a magnetic field with shorter period and a magnetic field of the deflection poles being smaller than in the case of the wiggler, so that interference effects occur in the synchrotron radiation. Due to the interference effects, the synchrotron radiation has a discontinuous spectrum and is emitted both horizontally as well as vertically in a small solid-angle element; i.e., the radiation is highly directional.
With suitable dimensioning all above-mentioned synchrotron EUV radiation sources, provide EUV radiation, for example, of 13 or 11 nm with sufficient power for EUV lithography.
Concerning synchrotron radiation, reference is made to Ernst Eckhart Koch, xe2x80x9cHandbook of Synchrotron Radiationxe2x80x9d, 1983, Elsevier-Science, New York, the disclosure of this publication is included herein by reference.
Since the radiation sources according to the invention are characterized by a beam divergence that is smaller than 5 mrads, at least in one plane, advantageously the system comprises means for broadening the beam, for example, a collector system.
In an advantageous embodiment, diverging mirrors or scanning mirrors, which are moved for illuminating a surface can be provided as means for broadening a beam.
Since field and aperture of the light source are insufficient for filling or illuminating field and aperture in the reticle plane, the illumination system according to the invention contains at least one mirror or lens with raster elements for producing a plurality of secondary light sources, which are distributed uniformly in the diaphragm plane. Since the geometric dimensions of the raster elements of the first mirror or of the first lens determines the form of the illuminated field in the reticle plane, field raster elements are formed preferably in an rectangular shape in the case of an arc-shaped scanning slit. The raster elements of the first mirror, which are also designated as field raster elements, are designed in such a way that their optical effect is to form an image of the light source in the diaphragm plane, so-called secondary light source. If the extension of the light source is small, for example, approximately point-like, as in the case of an undulator source, then the extension of the secondary light source is also small, and all light beams approximately pass through one point. In each plane after the diaphragm plane then an image of the field raster elements is formed, whereby the magnification is given by the ratio of the distance diaphragm-reticle to the distance field raster element-diaphragm. The raster elements are tilted in such a way that the images of the field raster elements are superimposed at least partially in the reticle plane.
The secondary light sources are advantageously imaged into the entrance pupil of the objective with a field mirror or a field lens, whereby the field lens or field mirror forms the arc-shaped field by controlling the distortion. The magnification of the field raster element imaging is not modified thereby.
In the case of extended light sources, as, for example in case of a bending magnet, the secondary light sources are extended; therefore the images of the field raster elements in the reticle plane are not sharp. A sharp image can be achieved in such a system, if one provides a second mirror or lens with raster elements, i.e., a so-called double facetting, wherein the raster elements of the second mirror or lens, the so-called pupil raster elements, are located on the site of the secondary light sources.
In systems with two mirrors with raster elements, the form of the raster elements of the second mirror, i.e., the pupil raster elements, is adapted to the shape of the secondary light sources and thus differs from the form of the first raster elements, i.e., the field raster elements. It is particularly preferred if the pupil raster elements are round, if the light source is also round in shape.
It is particularly preferred that the first mirror with raster elements is illuminated in a round manner or rotation-symmetrically, since then a uniform distribution of the secondary light sources in the diaphragm plane can be achieved with an appropriate distribution.
If the illumination of the first mirror is not round, but, for example, rectangular, then the desired round illumination of the entrance pupil of the objective is achieved by double facetting such a system.
The optical elements situated after the mirrors with raster elements serve for imaging the diaphragm plane of the illumination system in the entrance pupil of the projection objective and to form the arc-shaped field. Further, they serve for forming the illumination distribution according to the requirements of the exposure process.
It is particularly preferred, that the optical elements comprise grazing-incidence mirrors with an angle of incidence xe2x89xa620xc2x0. In order to minimize the light losses associated with each reflection, it is advantageous if the number of field mirrors its kept small. Embodiments with at most two field mirrors are particularly preferred.
A numerical example will be given below, from which it is obvious that increasing the waveguide value, i.e. the Lagrange optical invariant, for example, in the case of an undulator source is necessary.
If one requires an aperture in the wafer plane of NAwafer=0.1-0.25, then this means an aperture in the reticle plane of NAreticle=0.025-0.0625 in the case of 4:1 systems. If the illumination system will illuminate this aperture homogeneously and independently of the field up to a filling degree of "sgr"=0.6, then the EUV source must make available the following 2-dim waveguide value (LLW), i.e., the Lagrange optical invariant or etendu.
LLWillumination="sgr"2LLWobj=0.149 mm2xe2x88x920.928 mm2.
The waveguide value LLW, i.e., the Lagrange optical invariant, is defined generally as follows:
LLW=xxc2x7yxc2x7NA2+Axc2x7NA2,
whereby A is the illuminated surface. In the reticle plane, A amounts to, e.g., 110 mmxc3x976 mm.
An undulator source will be considered as a light source for the EUV illumination system according to the invention, in a first form of embodiment.
The waveguide value, i.e., the Lagrange optical invariant or etendu, for the undulator source, can be estimated according to a simplified model, assuming a homogeneous surface radiator with diameter
xc3x8=1.0 mm and aperture NAundulator=0.001 with
LLW=Axc2x7NA2                               A          undulator                =                  xe2x80x83                ⁢                  π          ·                                    (                              xe2x88x85                /                2                            )                        2                                                  =                  xe2x80x83                ⁢                  0.7850          ⁢                      xe2x80x83                    ⁢                      mm            2                                                            NA          undulator                =                  xe2x80x83                ⁢        0.001            
so that
LLWundulator=Axc2x7NA2=0.00000079 mm2=7.9exe2x88x9207 mm2.
As can be seen from this rough estimation, the waveguide value of the undulator source is disappearingly small in comparison to the required waveguide value.
The waveguide value, i.e., the Lagrange optical invariant, can be increased by providing distributed secondary light sources to the necessary amount in the entrance pupil of the objective. For this purpose, the first mirror is designed with raster elements. The illumination of the entrance pupil of an objective is defined by the filling factor. The following applies:       Filling    ⁢          xe2x80x83        ⁢    factor    ⁢          :        ⁢          xe2x80x83        ⁢    σ    =            r      illumination              R              objective        ⁢                  xe2x80x83                ⁢        aperture            
wherein Robjective aperture is the radius of the entrance pupil of the objective, and
rillumination is the radius of illumination of the field raster element plate in the case of annular illumination.
With "sgr"=1.0, the entrance pupil is completely filled; "sgr"=0.6 corresponds to an underfilling.
Since the partial pupils have sharp intensity peaks due to the small waveguide value of the undulator source, it is advantageous if these are smeared by means of xe2x80x9cwobblingxe2x80x9d field mirrors, whereby the-field illumination should remain unaffected. Thus, it is advantageous to introduce a wobbling field mirror as close as possible to the reticle plane.
An estimation for the angular region to be varied by the wobbling field mirror or by the periodically moving field mirror will be given below. If one assumes for the aperture in the reticle plane NAret=0.025 and the distance of the partial pupils amounts to approximately 0.005 for xcex94NA, due to the parceling, then the angular region to be varied should fie in the order of magnitude of approximately xc2x12.5 mrads. An example of a wobbling field mirror would be a toroidal mirror with a size of 160xc3x97170 mm as well as a local dynamic gradient of xc2x12 mrads in the x and y directions with a stability of xc2x10.1 mrad.
A smearing can be achieved not only by means of movable so-called wobbling field mirrors, but also by dynamic deformation of the mirror surface.
In order to achieve a high scanning uniformity, the use of active lenses or mirrors for the optical elements can be advantageous.
Since the manufacture of field raster elements with a high aspect ratio of 20:1, for example, is difficult, in order to reduce the aspect ratio of field raster elements, it can be of advantage that these raster elements are of astigmatic shape. The secondary light sources are thus broken down into tangential and sagittal secondary sources, which lie in the tangential and sagittal diaphragm planes.
Whereas the system for wavelengths in the EUV region, as described above, is designed purely reflectively, i.e., exclusively with mirror components, a use is also conceivable for 193-nm or 157-nm systems. In such a case, refractive components such as lenses are used.
The systems described herein are particularly of interest for 193-nm or 157-nm systems, because they use only a few optical components and the optical elements have high absorptions at these wavelengths.
Advantageous configurations of the invention are the subject of the subclaims.